chore: consolidate bigtable mypy config and update comments

Author: chalmerlowe

mypy.ini

@@ -95,17 +95,16 @@ ignore_missing_imports = True
 ignore_missing_imports = True
 
 [mypy-google.cloud.bigtable.*]
+# Broadly ignore errors for the older, legacy/handwritten parts of the library
 ignore_errors = True
 
-[mypy-google.cloud.bigtable.*]
+[mypy-google.cloud.bigtable.data.*]
+# Stricter rules for the newer parts of the library
 check_untyped_defs = True
 warn_unreachable = True
 disallow_any_generics = True
 ignore_errors = False
 
-[mypy-google.cloud.bigtable.data.*]
-ignore_errors = False
-
 [mypy-google.cloud.bigtable_admin.*]
 ignore_errors = True